[RFC 8.3 3/3] Avoid a crash in source_cache::extract_lines

[Pedro Alves]

On 03/08/2019 09:04 PM, Tom Tromey wrote:
> If the first requested line is larger than the number of lines in the
> source buffer, source_cache::extract_lines could crash, because it
> would try to pass string::npos" to string::substr.
> 
> This patch avoids the crash by checking for this case.

Can you clarify how can first_pos end up as npos?  Is that a bug in the
caller, or is it normal?  The documentation doesn't seem to allow for that:

  /* Get the source text for the source file in symtab S.  FIRST_LINE
     and LAST_LINE are the first and last lines to return; line
     numbers are 1-based.  If the file cannot be read, false is
     returned.  Otherwise, LINES_OUT is set to the desired text.  The
     returned text may include ANSI terminal escapes.  */

> 
> gdb/ChangeLog
> 2019-03-08  Tom Tromey  <tromey@adacore.com>
> 
> 	* source-cache.c (source_cache::extract_lines): Handle case where
> 	first_pos==npos.
> ---
>  gdb/ChangeLog      | 5 +++++
>  gdb/source-cache.c | 2 ++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/gdb/source-cache.c b/gdb/source-cache.c
> index 27a0ade959c..b5d0d6cb7fc 100644
> --- a/gdb/source-cache.c
> +++ b/gdb/source-cache.c
> @@ -98,6 +98,8 @@ source_cache::extract_lines (const struct source_text &text, int first_line,
>  	{
>  	  if (pos == std::string::npos)
>  	    pos = text.contents.size ();
> +	  if (first_pos == std::string::npos)
> +	    first_pos = text.contents.size ();
>  	  *lines = text.contents.substr (first_pos, pos - first_pos);
>  	  return true;
>  	}
> 

Thanks,
Pedro Alves